A conventional electromagnetic digitizer 100 as shown in FIG. 1 includes a printed circuit board 102 upon which are printed a number of conductive planar sensing loops, such as sensing loops 103 through 108. Sensing loops 103-105 are printed on one side of the printed circuit board 102 and sensing loops 106-108 are printed on the other side of the printed circuit board 102. Each sensing loop produces an electrical signal with an amplitude and phase that are determined by the position of a stylus 101 with respect to the printed circuit board 102. The sensing loops 103-105 are arranged into two sets, one for detecting the X position of the stylus 101, and the other for detecting the Y position of the stylus 101. Typically, the electromagnetic digitizer 100 has thirty-two X sensing loops and twenty-four Y sensing loops, although these numbers may vary. The stylus 101 typically generates a magnetic field that influences currents in the sensing loops 103-108. An analysis of the electrical signals in the sensing loops 103-108 provides sufficient information to determine the location of the stylus 101 relative to the printed circuit board 102.
These types of digitizers require plated through-holes in the printed circuit board 102. Examples of such through-holes are shown as through-holes 109. The through-holes 109 provide conductive paths through the printed circuit board 102 from one surface to the other opposing surface. Because the sensing loops corresponding to a particular sensing axis (the X sensing loops 103-105 or the Y sensing loops 106-108) are interleaved and overlapping, the through-holes 109 allow the sensing loops 103-108 to cross each other without short circuiting. Because of the need to prevent short-circuiting, nobody has made such electromagnetic digitizers using a single layer. Instead, diagonal paths (the paths that pass through the through-holes 109) that form the top side of each of the sensing loops are placed on the opposite surface of the printed circuit board 102. The end paths are diagonally shaped to reduce the number of through-holes necessary and the complexity of the interleaved sensing loop shape.
A problem with requiring plated through-holes is that they require additional manufacturing, which consumes time and money. Another problem is that the through-hole requirement limits the types of materials that may be used to fabricate the electromagnetic digitizer 100; those materials that cannot support plated through-holes, such as glass, cannot be used to fabricate the electromagnetic digitizer 100.
In addition, the diagonal configuration of the ends of the sensing loops consumes excessive space on the printed circuit board 102 that is not itself usable for sensing the position of the stylus 101. Thus, the extended points of the diagonal paths may be considered overhead, and to provide space for the diagonal traces and to minimize nonlinearity, the printed circuit board 102 must extend beyond the active area of the digitizer. Thus, when such a digitizer is integrated with a liquid crystal display to create a stylus-sensitive display, for example, the digitizer must be larger than the displayable area. This overhead space is undesirable to the user because it makes the product larger than apparently necessary, and it creates considerable manufacturing difficulties as well.
FIG. 2 provides an electrical schematic of a portion of the electromagnetic digitizer 100, shown in this case with eleven sensing loops L1 through L11. Only one sensing axis is shown in FIG. 2. The sensing loops L1-L11 are selectively coupled to a processing unit 201 that determines the position of the stylus 101 relative to the sensing loops L1 through L11. The details of the signal processing by the processing unit 201 vary among manufacturers. Wacom Technology Corporation provides digitizers that excite a resonant circuit in the stylus and then terminate the exciting signal. The amplitude of the exponentially decaying oscillatory field is measured. Fine Point Innovations uses a stylus with a battery that runs an oscillator to produce a steady signal. In any event, the amplitude and the phase of the signal from each sensing loop L1-L11 is measured by sequentially connecting the sensing loops to the processing unit 201 that uses analog multiplexors. FIG. 2 represents the multiplexors as switches S1 through S11. Depending upon which switch is closed, a different one of the sensing loops L1-L11 is activated to form a circuit between the processing unit 201 and ground. Only one of the switches S1 through S11 is closed at a time.
As previously discussed, a problem with the design of FIGS. 1 and 2 is that the sensing loops must cross one another, which translates into through-hole manufacturing problems. Such crossings where through-holes are necessary are indicated by way of examples as broken circles showing where sensing loop L1 overlaps with sensing loops L2 through L5 at their diagonal paths. The points where the various sensing loops overlap cannot electrically touch one another or else undesired short circuits will develop.
An improved electromagnetic digitizer design is needed to overcome at least one or more of the above-identified problems.